1. Field of the Invention
The present invention generally relates to a laser radar apparatus. More specifically, the present invention is directed to a coherent laser radar apparatus capable of measuring physical information such as a distance of a target, a target velocity, a density distribution of a target, and a velocity of a target, while using as a light source a laser oscillated with having a single wavelength, for instance, is directed to such a coherent laser radar apparatus in which transmitted radiation is modulated by a pseudo-random sequence (PN code), while employing a CW laser radiation source oscillated with having a single wavelength. Furthermore, the present invention is related to a compact/highly reliable coherent laser radar apparatus mounted on a mobile object such as an aircraft and a vehicle. Also, the present invention is related to a system constituted by integrating an optical space communication apparatus with a coherent laser radar apparatus.
2. Description of the Related Art
There are various coherent laser radar apparatus using light waves (lasers) and also pulse Doppler radars with employment of microwaves and millimeter-waves, which may function as apparatuses designed to measure various physical information such as distances, velocities, density distributions, velocity distributions as to targets. Among these radar apparatuses, the pulse Doppler radars are capable of measuring targets over wide bands and also over long distances, whereas the coherent laser radar apparatuses are capable of realizing both high spatial resolution and high speed resolution, due to differences in frequencies under use. There is such a definition as to a target. That is, a single target which owns a certain dimension and also a boundary surface functioning as a reflection surface and a scattering surface is referred to as a xe2x80x9chard targetxe2x80x9d, for instance, an aircraft and a vehicle. Also, in such a case that scattered radiation is synthesized with each other to constitute received radiation and this scattered radiation is derived from a large number of very fine scattering members which are distributed in a certain space, this spatially distributed target is referred to as a xe2x80x9csoft targetxe2x80x9d.
While measuring soft targets such as a wind velocity and a wind velocity distribution, a pulse Doppler radar measures a Doppler shift of an echo of a scattering object mainly including particles of rain droplets, fog, cloud in the atmosphere, and then acquires a wind velocity. As a result, in fine weather where no particles of rain droplets, fog, and cloud are present in the atmosphere, echoes having sufficiently large intensitys cannot be acquired. Therefore, there is such a drawback that the pulse Doppler radars cannot measure clean-air turbulence.
Since a coherent laser radar apparatus with employment of laser radiation may acquire a sufficiently high scattering intensity even from aerosol in the atmosphere, this laser radar apparatus may measure a wind velocity and a wind velocity distribution even in fine weather. As a result, such a coherent laser radar apparatus may be expected as an obstacle sensing apparatus capable of sensing various obstacles including air turbulence, which may be mounted on an aircraft, or in an airport.
As a coherent laser radar apparatus, there are a radar apparatus in which a pulse laser oscillated having a single frequency is employed as a light source, and another radar apparatus in which a CW laser is employed as a light source.
FIG. 29 schematically shows a structural diagram of a coherent laser radar apparatus in which the injection-seeded pulsed laser apparatus is employed a light source, which is disclosed in U.S. Pat. No. No. 5,237,331 issued to Sammy W. Henderson at al.
In FIG. 29, reference numeral 1 shows a laser radiation source oscillated having a single frequency, reference numeral 2 indicates a first optical dividing device, reference numeral 3 represents a frequency shifter, reference numeral 4 is an injection-seeded pulsed laser, reference number 5 shows a beam splitter, reference numeral 6 indicates a xc2xc wavelength plate, reference numeral 7 denotes a transceiver optics, reference numeral 8 represents a scanning optics, reference numeral 9 indicates a first combining device, and reference numeral 10 shows a first photodetector. Also, reference numeral 11 is a second optical dividing device, reference numeral 12 shows a third optical dividing device, reference numeral 13 is a second combining device, reference numeral 14 is a second photodetector, reference numeral 15 shows an A/D converter, reference numeral 16 represents a signal processing apparatus, reference numeral 17 shows an adjusting mechanism constructed of a piezoelectric transducer of a resonator length of the injection-seeded pulse laser 4, reference numeral 18 shows a control circuit of the adjusting mechanism, reference numeral 19 shows laser radiation emitted from the laser radiation source 1, and reference numeral 20 denotes seed light. Also, reference numeral 21 indicates pulse laser radiation, reference numeral 22 is an optical axis of the transmitted/resceived radiation, reference numeral 23 shows transmitted radiation, reference numeral 24 represents received radiation, reference numeral 25 shows local radiation, and furthermore, reference numeral 26 shows mixture light made by combining the received radiation 24 with the local radiation 25.
Next, operations will now be made.
The laser radiation 19 supplied from the laser radiation source 1 oscillated having a single wavelength xe2x80x9cf0xe2x80x9d is subdivided by the first optical dividing device 2 into two laser radiations. One laser radiation is employed as the local radiation 25, and the other laser radiation is shifted by the frequency shifter 3 by a frequency xe2x80x9cfIFxe2x80x9d to become such laser radiation whose frequency is increased only by this frequency fIF. This frequency-shifted laser radiation is supplied as the seed radiation 20 to the injection-seeded pulsed laser 4. The injection-seeded pulsed laser 4 oscillates a laser pulse having a single frequency (single wavelength) in an axial mode having such a frequency located at the nearest frequency of the seed radiation 20.
Since the laser pulse radiation 21 emitted from the injection-seeded laser pulse 4 is linear-polarized, this laser pulse radiation 21 is reflected by the beam splitter 5. After this reflected laser pulse radiation is converted into the circular-polarized laser radiation by the xc2xc wavelength plate 6, this circular-polarized laser radiation is traveled through both the transceiver optics 7 and the scanning optics 8, and then is projected toward the target. The scattering radiation originated from the target is received via such an optical path opposite to the above-explained optical path of the transmitted radiation.
The received radiation 24 is traveled via both the scanning optics 8 and the transceiver optics 7, and then, is processed by the xc2xc wavelength plate 6 to become linear-polarized laser radiation which is shifted by 90 degrees with respect to the polarizing plane of the laser pulse radiation 21. Then, the linear-polarized laser radiation passes through the beam splitter 5 so as to be conducted to the first combining device 9. In the first combining device 9, the received radiation 24 is mixed with the local radiation 25. The combined radiation 26 is coherent-detected in the first photodetector 10. The signal detected by the first photodetector 10 is sampled by the A/D converter 15, and the signal processing apparatus 16 extracts the distance of the target from the temporal waveform of the intensity signal, and also extracts the velocity of the target from the Doppler signal.
As previously explained, since the injection-seeded pulse laser 4 oscillates the pulse having the single frequency in the axial mode having such a frequency located at the nearest frequency of the seed light 20, a difference between the frequency of the pulse laser radiation 21 and the frequency of the local radiation 25 must be monitored in order to obtain the accurate Doppler signal. To this end, a portion of the pulse laser radiation 21 and a portion of the local radiation 25 are derived by the second optical branch 11 and the third optical branch 12. After the derived light portions are mixed with each other by the second optical combining device 13, the combined radiation is coherent-detected by the second photodetector 14. Similar to the received radiation, the detected laser radiation is sampled by the A/D converter 15 so as to calculate a. frequency difference between the pulse laser radiation 21 and the local radiation 25 by the signal processing apparatus 16. Assuming now that the frequency of the local radiation 25 is selected to be xe2x80x9cfOxe2x80x9d, the respective frequencies xe2x80x9cfSxe2x80x9d, xe2x80x9cfTxe2x80x9d, xe2x80x9cfRxe2x80x9d, xe2x80x9cfMxe2x80x9d, and xe2x80x9cfsigxe2x80x9d of the seed light, the pulse laser radiation, the received radiation, the frequency monitor signal, and the recept on signal are expressed by the following formulae:
xe2x80x83fs=f0+fIF
fT=fs+xcex94f
fR=fT+fd
fM=fIF+xcex94f
fsig=fM+fd
In the formulae, symbol xe2x80x9cxcex94fxe2x80x9d indicates the frequency difference between the laser pulse radiation and the seed light, and symbol xe2x80x9cfdxe2x80x9d shows the Doppler frequency of the target. Since the difference between the reception signal xe2x80x9cfsigxe2x80x9d and the frequency monitor signal xe2x80x9cfMxe2x80x9d is calculated, the Doppler frequency xe2x80x9cfdxe2x80x9d of the target can be acquired.
To realize the injection seeding operation under stable condition, the resonator length of the pulsed laser 4 is adjusted by the adjusting mechanism 17 made of the piezoelectric transducer element. This adjusting mechanism 17 constructed of the piezoelectric transducer element is controlled by the control circuit 18. The signal processing apparatus 16 supplies the value of xe2x80x9cxcex94fxe2x80x9d obtained from the monitor signal for the frequency difference between the laser pulse radiation 21 and the local radiation 25 to the control circuit 18. Under control of the control circuit 18, the resonator length of the pulsed laser 4 is adjusted by the adjusting mechanism 17 constructed of the piezoelectric transducer element in order that the value of xe2x80x9cxcex94fxe2x80x9d becomes smaller than or equal to the set value, or otherwise becomes zero.
As previously explained, the pulse laser radiation oscillated in a single mode (single wavelength) can be obtained under stable condition.
In the coherent laser radar apparatus by employing the pulse laser, two sets of lasers are required, namely both the pulsed laser and the CW laser functioning as the local radiation source are needed. Also, in order to obtain such a pulse width larger than or equal to several hundreds ns in the pulsed laser, the resonator length is longer than or equal to 1 m, and also since the high speed switching operation is carried out in the Q switching operation, the electromagnetic noise is produced. This fact may cause the drawback when this coherent laser radar apparatus is used in the field of the aircraft mount type radar apparatus which necessarily requires the compact, highly reliable, and low electromagnetic noise characteristics.
A coherent laser radar apparatus in which a coherent CW laser is employed in a light source can solve the above-explained drawback of the coherent laser radar apparatus with employment of the pulsed laser, and also may probably realize both an arbitrary distance resolution and a velocity resolution by modulating transmitted radiation.
FIG. 30 is a structural diagram for indicating such a coherent CW laser radar apparatus in which the CW laser oscillating a laser radiation having a single wavelength is employed as the light source, as disclosed in Japanese Patent Application Laid-open No. Hei 2-284087 field by HIRANO et al.
In the arrangement shown in FIG. 30, the laser radiation emitted from the CW laser oscillator 31, having a single wavelength is subdivided into two sets of the laser radiation by the optical distributor 32. One of the subdivided laser radiation is modulated by the optical modulator 34 which performs the modulating operation based upon the pseudo-random modulation signal generated by the sequence generator 33. The optically modulated laser radiation is propagated via the pollarizer 35 and the xc2xc-wavelength plate 36, and then is projected from the transceiver optics 37 toward the target 39 as the transmitted radiation 38. The transmitted radiation 38 is scattered, or reflected by the target 39. Then, a portion of either the scattered radiation or the reflection light is received by the transceiver optics 37 as the received radiation 40. The received received radiation 40 is propagated via the xc2xc-wavelength plate 36, and then, is separated from the transmitted radiation 38 in the polarizer 35 to be conducted to the wavelength division multiplexer 41. 
The other laser radiation which is emitted from the laser oscillator 31 and then is subdivided by the optical distributor 32 is employed as the local radiation used in the optical heterodyne detection. The local radiation is propagated via the reflection mirror 42 to the frequency shifter 43, and then the optical frequency of this local radiation is shifted by the intermediate frequency xe2x80x9cfIFxe2x80x9d. Thereafter, the frequency-shifted local radiation is processed by the xc2xd-wavelength plate 44 in such a manner that the polarization plane of this local radiation is rotated so as to make this polarization plane coincident with the polarization surface of the received radiation 40 in the second optical separator 41. Then, the resulting local radiation is combined with the received radiation 40 in the second optical separator 41. The combined radiation between the local radiation and the received radiation 40 is optical-heterodyne-detected in the PD45 corresponding to the photodetector. The reception signal supplied from the PD45 is amplified by the amplifier 46, and then, the amplified reception signal is supplied via the band-pass filter 47 to the correlating device 48. 
In the correlating device 48, the reception signal is multiplied by the pseudo-random modulation signal to acquire the correlation. This pseudo-random modulation signal is produced by modulating the transmitted radiation to which arbitrary delay time td is applied by the variable delay circuit 28. When the target 39 is such a hard target having a sufficiently high reflectance, a peak value of output power from the correlating device 48 may be obtained by the power measuring device 49 in such a case that the reciprocation time xe2x80x9ctrxe2x80x9d of the received radiation up to this hard target 39 becomes equal to the delay time xe2x80x9ctdxe2x80x9d. Assuming now that the Doppler frequency of the received radiation while the target is moved is selected to be xe2x80x9cfdxe2x80x9d, a frequency difference of (fIFxe2x88x92fd) is acquired as the output frequency of the correlating device 48 by the frequency discriminator 26.
As a consequence, since the delay time td is swept by the control apparatus 27 over the measuring area, the distance information of the target may be acquired from the power measuring device 49, and also the velocity information of the target may be acquired from the frequency discriminator 26.
As previously explained, the coherent laser radar apparatus with employment of the pulsed laser, as shown in FIG. 29, owns the following drawbacks. That is to say, two sets of lasers are required, namely both the pulsed laser and the CW laser functioning as the local radiation source are needed. Also, the resonator length of pulsed laser longer than or equal to 1 m, making the apparatus large. Further, since the high speed switching operation is carried out in the Q switching operation, the electromagnetic noise is produced. In addition, since the laser pulse having the high peak power is employed, there are other drawbacks that the reliabilities as to the optical components are lower than the reliability of the CW laser radiation, and the laser radiation is traveled within the apparatus by way of the space propagation manner. This fact may cause the drawback when this coherent laser radar apparatus is used in the field of the aircraft mount type radar apparatus that necessarily requires the compact, highly reliable, and low electromagnetic noise characteristics.
Also, as shown in FIG. 30, the laser radar apparatus in which while the CW laser apparatus is employed, the modulating operation is carried out by using the pseudo-random signal, owns the below-mentioned drawbacks:
(1). The signal light is traveled within this laser radar apparatus by way of the space propagation manner. In the optical heterodyne detection, the wave front of the received radiation must be made coincident with the wave front of the local radiation on the photodetector. In order to acquire the reception signal under stable condition, a large number of optical elements provided in the optical path are required to be fixed in such a manner that these optical elements can have high rigidness and this high rigidness is not varied. As a consequence, the laser radar apparatus becomes complex and the reliability thereof is lowered. Furthermore, the spatial free degrees of this laser radar apparatus are restricted when this laser radar apparatus is mounted on the mobile object such as an aircraft.
(2). Since the CW laser radiation source oscillating a laser radiation having a single wavelength is constructed of a solid-state laser, output power of this solid-state laser is not so high, namely on the order of several tens of mW to several hundreds of mW, so that a sufficiently high S/N ratio cannot be obtained.
(3). The monitoring time during which the S/N ratio can be improved in a linear manner is restricted by both the line width of the CW laser radiation source and the frequency dispersion given to the reception signal based on the target. The maximum monitoring time is equal to approximately the inverse number thereof. Since the line width of the CW laser radiation source oscillated, having a single wavelength is equal to several KHz, the accumulation time by the correlating process operation is limited to less than 1 ms. As a result, even when the accumulation time is prolonged, the higher S/N ratio cannot be obtained.
(4). In such a case that the reflected/scattered radiation (internal reflection light) occurred in the window formed in the transceiver optics, or the mobile object housing is larger than the signal received from the target, even when the spread spectrum process operation is carried out as the non-correlative component by the pseudo-random modulating manner, the spectrum intensity cannot be made sufficiently higher than the peak intensity of the target, and therefore, the high S/N ratio measurement cannot be carried out.
(5). In order to obtain the velocity distribution, the delay time xe2x80x9ctdxe2x80x9d must be swept, so that both the variable delay device and the control circuit thereof are required.
Furthermore, in such a system that the coherent laser radar apparatus is mixed with the optical space communication apparatus, two sets of plural optical systems and also two sets of optical components are necessarily provided. As a consequence, the entire apparatus is made complex, resulting in another drawback.
FIG. 31 indicates the laser radar apparatus disclosed in Japanese Examined Patent Publication No. Sho 64-2903 filed by TAKEUCHI et al., which corresponds to such a laser radar apparatus capable of acquiring the distance information of the target by modulating the transmitted radiation by the pseudo-random signal by employing the CW laser in the optical light source.
In FIG. 31, reference numeral 1 shows a transmission unit, reference numeral 2 indicates a reception unit, reference numeral 3 represents a transmitter optics, and reference numeral 4 denotes a receiver optics. The transmission unit 1 is arranged by the laser oscillator 5 for oscillating CW light, the modulator 6, and the sequence generator 7 for generating the pseudo-random signal (for example, M sequence and Barker Number series). Also, the reception unit 2 is arranged by the photodetector 8, the delay correlator 9, and the display recording unit 10.
Next, a description will now be made of operations of the laser radar apparatus with the arbandment shown in the drawing.
The CW laser radiation oscillated from the laser oscillator 5 is modulated by the pseudo-random signal (one sequence length=M bits, and time width per 1 bit=xcfx84) generated from the sequence generator 7, and then the modulated CW light is projected as the transmitted radiation from the transmitter optics 3 into the atmosphere. It is now assumed that a distance over which a ratio is changed from 0 to plus is set as Rm. At this ratio, the transmitted radiation is included in the field of the receiver optics 4. This distance Rm corresponds to the minimum measuring distance of this laser radar apparatus, and therefore, this laser radar apparatus can measure such a target located further than this distance Rm.
The reflection radiation reflected from the target is received by the receiver optics 4, is detected by the photodetector 8, and then is converted into the electric signal. This reception signal is recorded, and then, the delay correcting device 9 performs the correlating process operation is such a manner that this recorded reception signal is multiplied by the pseudo-random signal derived from the sequence generator 7 and multiplied by the time delay td. Since this delay time td is adjusted, the information of the reflection intensity of such a distance that the reciprocation time tr of the received radiation is made equal to the delay time td can be measured based on the distance resolution c xcfx84/2 (symbol xe2x80x9ccxe2x80x9d indicates light velocity). As a result, since the delay time td is swept within the measurement region, when the target is a hard target, a position of this hard target can be measured, whereas when the target is a soft target, a spatial distribution of this soft target can be measured.
While the CW laser oscillating a laser radiation having a single frequency is employed as the light source, since the heterodyne detection is carried out in the reception unit, the Doppler shift amount of the target is detected, so that not only the distance information of the target, but also the velocity information thereof can be acquired.
A coherent laser radar apparatus in which a coherent CW laser is employed in a light source can solve the above-explained drawback of the coherent laser radar apparatus with employment of the pulsed laser, and also may probably realize both an arbitrary distance resolution and a velocity resolution by modulating transmitted radiation.
FIG. 32 is a structural diagram for indicating such a coherent CW laser radar apparatus in which the CW laser oscillating a laser radiation having a single wavelength is employed as the light source, as disclosed in Japanese Patent Application Laid-open No. Hei 2-284087 filed by HIRANO et al.
In the arbandment shown in FIG. 32, the laser radiation emitted from the CW laser oscillator 31, having a single wavelength is subdivided into two sets of the laser radiation by the optical distributor 32. One of the subdivided laser radiation is modulated by the optical modulator 34 which performs the modulating operation based upon the pseudo-random modulation signal generated by the sequence generator 33. The optically modulated laser radiation is propagated via the polarizer 35 and the xc2xc-wavelength plate 36, and then is projected from the transceiver optics 37 toward the target 39 as the transmitted radiation 38. The transmitted radiation 38 is scattered, or reflected by the target 39. Then, a portion of either the scattered radiation or the reflection radiation is received by the transceiver optics 37 as the received radiation 40. The received received radiation 40 is propagated via the xc2xc-wavelength plate 36, and then, is separated from the transmitted radiation 38 in the polarizer 35 to be conducted to the second optical separator 41.
The other laser radiation that is emitted from the laser oscillator 31 and then is subdivided by the first optical separator 32 is employed as the local radiation used in the optical heterodyne detection. The local radiation is propagated via the reflection mirror 42 to the frequency shifter 43, and then the optical frequency of this local radiation is shifted by the intermediate frequency xe2x80x9cfIFxe2x80x9d. Thereafter, the frequency-shifted local radiation is processed by the xc2xd-wavelength plate 44 in such a manner that the polarizationc plane of this local radiation is rotated so as to make this polarization plane coincident with the polarization plane surface of the received radiation 40 in the second optical separator 41. Then, the resulting local radiation is combined with the received radiation 40 in the second optical separator 41. The combined radiation between the local radiation and the received radiation 40 is optical-heterodyne-detected in the PD45 corresponding to the photodetector. The reception signal supplied from the PD45 is amplified by the amplifier 46, and then, the amplified reception signal is supplied via the band-pass filter 47 to the correlating device 48. 
In the correlating device 48, the reception signal is multiplied by the pseudo-random modulation signal to acquire the correlation. This pseudo-random modulation signal is produced by modulating the transmitted radiation to which arbitrary delay time td is applied by the variable delay circuit 28. When the target 39 is such a hard target having a sufficiently high reflectance, a peak value of output power from the correlating device 48 may be obtained by the power measuring device 49 in such a case that the reciprocation time xe2x80x9ctrxe2x80x9d of the received radiation up to this hard target 39 becomes equal to the delay time xe2x80x9ctdxe2x80x9d. Assuming now that the Doppler frequency of the received radiation while the target is moved is selected to be xe2x80x9cfdxe2x80x9d, a frequency difference of (fIFxe2x88x92fd) is acquired as the output frequency of the correlating device 48 by the frequency discriminator 26.
As a consequence, since the delay time td is swept by the control apparatus 27 over the measuring area, the distance information of the target may be acquired from the power measuring device 49, and also the velocity information of the target may be acquired from the frequency discriminator 26.
FIG. 33 represents a structural diagram for explaining such a case that a soft target is measured.
As apparent from the structural diagram shown in FIG. 32, a signal processing unit subsequent to the correlating device 48 of the structural diagram shown in FIG. 33 is different. The operations of the modulation of the laser radiation and also the correlating operation between the transmission/received radiation and the pseudo-random modulation signal in FIG. 33 are identical to those of FIG. 32.
The correlation between the output signal from the correlating device 48 and a reception signal received from such a distance that the reciprocation time tr of the received radiation is made equal to the delay time td is made coincident, and this output signal has a large peak in the frequency (fIFxe2x88x92fd). As to all reception signals received from distances other than the above-explained distance, the output signal of the correlating device 48 becomes non-correlative. The frequency spectra of these reception signals is spread, and is extended into a wide frequency band.
In the signal processing unit shown in FIG. 33, the output signal of the correlating device 48 is converted into a digital signal by an A/D converter 101, this digital signal is processed by an FFT circuit 102 to acquire a frequency spectrum, and then, a signal processing circuit 103 detects such a peak that the frequency thereof becomes (fIFxe2x88x92fd). Based upon the frequency of the peak and the intensity thereof, a velocity of a target and a density distribution thereof may be calculated, and this target is located at such a distance that the reciprocation time tr of the received radiation is made equal to the delay time td.
A reception signal derived from a coherent CW laser radar apparatus when a soft target is present as a measuring target is expressed by the following formula (1):                               P          r                =                  η          ⁢                      xe2x80x83                    ⁢                      P            CW                    ⁢                      T            t                    ⁢                      T            r                    ⁢                                    ∫              0              ∞                        ⁢                                                            π                  ⁢                                      xe2x80x83                                    ⁢                                      D                    r                    2                                                                    4                  ⁢                                      R                    2                                                              ⁢                                                                    β                    ⁡                                          (                      R                      )                                                        ⁢                                                            T                      ⁡                                              (                        R                        )                                                              2                                                                    SRF                  ⁡                                      (                    R                    )                                                              ⁢                              xe2x80x83                            ⁢                              ⅆ                R                                                                        (        1        )            
In this formula (1), the respective symbols are defined as follows:
PCW: transmitted radiation output;
xcex2 (R): backward scattering coefficient of target (soft target);
Dr: aperture radius of receiver optics;
T(R): atmospheric transmittance;
Tt: transmittance of receiver optics;
SRF(R): signal reduction factor; and
R: distance.
Also, both the atmospheric transmittance T(R) and the signal reduction factor SRF(R) are expressed by the below-mentioned formulae (2) to (4):                                           T            ⁡                          (              R              )                                2                =                  exp          ⁡                      [                                          -                2                            ⁢                                                ∫                  0                  R                                ⁢                                                      α                    ⁡                                          (                                              R                        xe2x80x2                                            )                                                        ⁢                                      xe2x80x83                                    ⁢                                      ⅆ                                          R                      xe2x80x2                                                                                            ]                                              (        2        )                                          SRF          ⁡                      (            R            )                          =                  [                      1            +                                                            (                                      1                    -                                          R                      F                                                        )                                2                            ⁢                                                (                                                            kD                      r                      2                                                              8                      ⁢                      R                                                        )                                2                                      +                                          (                                                      D                    r                                                        2                    ⁢                                                                  S                        0                                            ⁡                                              (                        R                        )                                                                                            )                            2                                ]                                    (        3        )                                                      S            0                    ⁡                      (            R            )                          =                              [                                          Hk                2                            ⁢                                                ∫                  0                  R                                ⁢                                                                            C                      n                      2                                        ⁡                                          (                      R                      )                                                        ⁢                                                            (                                              1                        -                                                  xe2x80x83                                                ⁢                                                                              R                            xe2x80x2                                                    R                                                                    )                                                              5                      3                                                        ⁢                                      ⅆ                                          R                      xe2x80x2                                                                                            ]                                -                          3              5                                                          (        4        )            
In this formula 2, the respective symbols are defined as follows:
(Rxe2x80x2): atmospheric attenuation coefficient;
F: focal point distance of receiver optics; and
K=2xcfx80/xcex, H=2.914383,
C2n(R): atmospheric structural constant.
In such a case that a soft target distributed in a space, for example, air, is measured by using CW laser radiation, as illustrated in FIG. 34, this space is considered as a series connection of thin layers. A thickness of each of the thin layers is equal to xe2x80x9cxcex94Rxe2x80x9d, and these thin layers are positioned along an optical axis of transmitted radiation. In this case, the thickness xe2x80x9cxcex94Rxe2x80x9d is equal to distance resolution, and is expressed by the following formula (5):                               Δ          ⁢                      xe2x80x83                    ⁢          R                =                              c            ⁢                          xe2x80x83                        ⁢            τ                    2                                    (        5        )            
In this formula 5, the symbol is defined as follows:
xcfx84: time width of pseudo-random modulation signal per 1 bit (element).
Assuming now that each of these layers contains such a reflection member having the same reflectance as that owned by a target (soft target) of this own layer, the above-described formula (1) is given as follows:                               P          r                =                  η          ⁢                      xe2x80x83                    ⁢                      P            CW                    ⁢                      T            t                    ⁢                      T            r                    ⁢                                    π              ⁢                              xe2x80x83                            ⁢                              D                r                2                                      4                    ⁢                                    ∑                              i                =                1                            ∞                        ⁢                          xe2x80x83                        ⁢                                                                                β                    ⁡                                          (                                              R                        i                                            )                                                        ⁢                                                            T                      ⁡                                              (                                                  R                          i                                                )                                                              2                                                                                        R                    i                    2                                    ⁢                                      SRF                    ⁡                                          (                                              R                        i                                            )                                                                                  ⁢              Δ              ⁢                              xe2x80x83                            ⁢              R                                                          (        6        )            
In a pseudo-random modulation CW coherent rider laser radar, a reception signal of a single atmospheric layer is used as a signal component by executing a correlative process operation. A reception signal obtained from a K-th layer is expressed as follows:                               P                      r            ,            i                          =                  η          ⁢                      xe2x80x83                    ⁢                      P            CW                    ⁢                      T            t                    ⁢                      T            r                    ⁢                                    π              ⁢                              xe2x80x83                            ⁢                              D                r                2                                                    4              ⁢                              R                i                2                                              ⁢                                                    β                ⁡                                  (                                      R                    i                                    )                                            ⁢                                                T                  ⁡                                      (                                          R                      i                                        )                                                  2                                                    SRF              ⁡                              (                                  R                  i                                )                                              ⁢          Δ          ⁢                      xe2x80x83                    ⁢          R                                    (        7        )            
In general, an S/N ratio is expressed by a ratio of a signal intensity to system noise of a reception system such as s hot noise and thermal noise. The S/N ratio is expressed by the f following formula:                               S          N                =                                            η              f                        ⁢                          P                              r                ,                i                                                          hv            ⁢                          xe2x80x83                        ⁢            B                                              (        8        )            
In this formula 6, symbols are defined as follows:
xcex7f: electric filter coefficient; and
B: reception bandwidth.
The reception bandwidth is inverse proportional to time required to perform the correlation process operation, namely time (one sequence length time) equivalent to one sequence length of a pseudo-random modulation signal. As a result, one sequence length time required to obtain a necessary S/N ratio may be calculated based upon the formula (8).
Different from the case of the hard target, when a soft target is measured, scattered radiation from a plurality of space layers is received at the same time, as defined in the formula (6). Since the pseudo-random modulation is carried out, reception signals of non-correlative space layers which are not of interest are spectrum-spread. However, considering a summation of spread spectra of a plurality of space layers, a maximum value of reception signal intensitys should be sufficiently suppressed with respect to reception signals received from correlative space layers which are of interest.
FIG. 35 graphically represents a distance depending characteristic about reception intensitys of a coherent CW laser radar apparatus, which is calculated by employing the formula (7).
A position of a peak and a dimension of this peak are determined based upon a focal point distance xe2x80x9cFxe2x80x9d of a receiver optics, and an aperture diameter Dr of a receiver optics. As apparent from this drawing, it may be understood that there is an effective reception distance band from which an effective reception intensity is obtained. As a result, a reception signal received from a space layer existed in this effective reception distance band.
Under a condition indicated in FIG. 36, a frequency spectrum of a reception signal obtained by the FFT circuit 102 is acquired. While the effective reception distance band is selected to be 9 (see FIG. 36A) xc3x97equivalent to 9 bits of a pseudo-random modulation signal, one sequence length of the pseudo-random modulation signal obtained from the formula (8) is selected to be 31 bits (see FIG. 36B). As a result, one sequence length time equivalent distance (defined as such a distance over which signal can be reciprocated within time of one sequence length) is equal to 31xc3x97xcex94R. While the pseudo-random sequence (PN code) used in the pseudo-random modulation signal is assumed as a 31-bit M sequence, delay time is set in such a manner that a correlation can be made coincident with a single space layer existed in the effective reception distance band. A Doppler shift is present only in a correlative space layer.
FIG. 37 represents a calculation result of a frequency spectrum of a reception signal obtained by the FFT circuit 102 under this condition.
An abscissa of FIG. 37 represents a frequency where an intermediate frequency fIF is used as a reference frequency. A summation of spread spectra of a plurality of non-correlative space layers constitutes such a signal which is extended to a wide frequency area, in which a spectrum waveform owned by a rectangular wave having a pulse width xe2x80x9cxcfx84xe2x80x9d is employed as an envelope line (fB=1/xcfx84 in this drawing). However, as apparent from this drawing, there are several peaks having large values, depending upon a sort of a pseudo-random modulation signal, a sequence length, and a time width. When these peak values are substantially equal to, or larger than such a peak value owned by a spectrum of a reception signal received from a correlative space layer in interest it is not possible to acquire the information about the correlative space layer in interest. In FIG. 37, the spectrum of the reception signal received from the correlative space layer owns a single and sharp peak at a frequency of xe2x80x9cxe2x88x92fdxe2x80x9d, but is hidden by a summation of spread spectra of a plurality of non-correlative space layers.
As explained above, because of the adverse influence caused by the summation of the spread spectra of a plurality of non-correlative space layers, there is such a drawback that the soft target cannot be measured with maintaining sufficiently high precision by employing the coherent CW laser radar apparatus.
Also, in the above-described coherent CW laser radar apparatus for modulating the transmitted radiation by using the pseudo-random sequence (PN code), since the monitoring time per 1 measuring operation is prolonged, the sufficiently high S/N ratio can be acquired even when the CW laser having the lower power than that of the pulse laser is employed. However, to this end, there are some possibilities that the pseudo-random sequence (PN code) whose sequence length would become several hundreds bits and would exceed several thousands bits may be employed.
FIG. 38 graphically shows a spectrum of a reception signal in such a case that a uniform soft target is measured by employing a pseudo-random sequence (PN code) whose sequence length is equal to 127 bits.
An abscissa of this graph indicates a frequency, in which a frequency (fIFxe2x88x92fd) of the below-mentioned reception signal is used as a reference frequency. This reception signal is received from a distance where reciprocation time xe2x80x9ctrxe2x80x9d of received radiation is made equal to delay time xe2x80x9ctd.xe2x80x9d At a frequency 0(fIFxe2x88x92fd), a sharp peak of correlated reception signals appears. Also, over a wide frequency band defined from xe2x88x92fB up to fB(=1/xcfx84;xcfx84: time width per 1 bit), such reception signals are distributed which are spectrum-spread from the non-correlative entire distance within the distance band from which the effective reception intensity can be acquired. When the soft target measurement is carried out, the peak value of the correlative reception signals must be made sufficiently large, as compared with the spectrum-spread reception signal derived from the non-correlative overall distance.
As explained, furthermore, in the conventional coherent CW laser radar apparatus for measuring the soft target, the monitoring time for one measuring operation is prolonged so as to obtain the sufficiently high S/N ratio. Also, the pseudo-random sequence (PN code) is employed in which the time equivalent to one sequence length corresponds to the monitoring time per one measuring operation. As a result, such a pseudo-random sequence (PN code) of a long sequence length may be employed.
On the other hand, a frequency analysis of a reception signal is required so as to obtain a Doppler frequency. To carry out a correlation process operation, data of a reception signal for a time period equivalent to one sequence length must be employed in order to analyze this frequency. As indicated in FIG. 33, in the case that both the A/D converter and the FFT are employed in the signal processing unit, the FFT process operation must be carried out by using a large number of A/D-converted data of the reception signals for the time equivalent to 1 sequence length. A total calculation amount of FFT calculation is direct proportional to approximately a squared value of a total number of used data. As a result, there is such a drawback that the calculation amount for the FFT calculation becomes very large.
The present invention has been made to solve the above-described conventional problems of these laser radar apparatuses, and therefore, has an object to provide a coherent laser radar apparatus having a high reliability, which can increase structural free degrees of optical elements, and also can readily assemble these optical structural elements.
Another object of the present invention is to provide such a coherent laser radar apparatus capable of increasing an S/N ratio, and also capable of suppressing an adverse influence caused by a summation of spread spectra of a plurality of non-correlative space layers. This S/N ratio is expressed by a ratio of a signal intensity to system noise of a reception system such as shot noise and thermal noise.
Furthermore, another object of the present invention is to provide a coherent laser radar apparatus capable of detecting a peak of a reception signal in high precision, which is received from a correlative distance.
To achieve the above-described objects, a coherent laser radar apparatus, according to a first aspect of the present invention, is featured by comprising: as an optical component, a CW laser oscillating a laser radiation having a single wavelength; a dividing means for dividing laser radiation derived from the CW laser; an optical modulator for modulating one of the laser radiation divided by the dividing means; an optical antenna for projecting the modulated laser radiation as transmitted radiation toward a target, and also for receiving scattered radiation from the target as received radiation; combining means for combining the other of the laser radiation divided by the dividing means as local radiation with the received radiation received from the optical antenna; and a photodetector for optical-heterodyne-detecting the combined radiation; and also comprising: as an electric component, a pseudo-random modulation signal generator for supplying a pseudo-random modulation signal to the optical modulator; a variable delay device for time-delaying a portion of the pseudo-random modulation signal generated from the pseudo-random modulation signal generator; a correlating device for multiplying the output signal of the photodetector by the pseudo-random modulation signal time-delayed by the variable delay device; and signal processing means for acquiring physical information such as a distance of the target and a velocity of the target based upon a intensity and a frequency of the output signal from the correlating device, and also the delay time set by said variable delay device; wherein: an optical path between the optical components is constituted by an optical fiber.
In the coherent laser radar apparatus according to the first aspect, an optical fiber amplifier having a gain in the vicinity of a wavelength of the modulated laser radiation is further provided in the optical path between the optical modulator and the optical antenna.
In the coherent laser radar apparatus according to the first aspect, the CW laser corresponds to a CW laser oscillating a laser radiation having a single wavelength, the oscillation wavelength of which is a 1.5 xcexcm band, whereas the optical fiber amplifier corresponds to an optical fiber amplifier with employment of an Er3+ ion-doped optical fiber having a gain in the 1.5 xcexcm band.
In the coherent laser radar apparatus according to the first aspect, the CW laser corresponds to a CW laser oscillating a laser radiation having a single wavelength, the oscillation wavelength of which is any one of a 1.06 xcexcm band, a 0.98 xcexcm band, and a 1.3 xcexcm band, whereas the optical fiber amplifier is an optical fiber amplifier with employment of an Nd3+ ion-doped optical fiber having a gain in any one of the 1.06 xcexcm band, 0.98 xcexcm band, and 1.3 xcexcm band.
In the coherent laser radar apparatus according to the first aspect, the CW laser corresponds to a CW laser oscillating a laser radiation having a single wavelength, the oscillation wavelength of which is a 1.3 xcexcm band, whereas the optical fiber amplifier corresponds to an optical fiber amplifier with employment of a Pr3+ ion-doped optical fiber having a gain in the 1.3 xcexcm band.
In the coherent laser radar apparatus according to the first aspect, the CW laser corresponds to a CW laser oscillating a laser radiation having a single wavelength, the oscillation wavelength of which is a 1 xcexcm band, whereas the optical fiber amplifier corresponds to an optical fiber amplifier with employment of a Yb3+ ion-doped optical fiber having a gain in the 1 xcexcm band.
In the coherent laser radar apparatus according to the first aspect, the CW laser corresponds to a CW laser oscillating a laser radiation having a single wavelength, the oscillation wavelength of which is a 2.1 xcexcm band, whereas the optical fiber amplifier corresponds to an optical fiber amplifier with employment of an Ho3+ ion-doped optical fiber having a gain in the 2.1 xcexcm band.
In the coherent laser radar apparatus according to the first aspect, the CW laser corresponds to a CW laser oscillating a laser radiation having a single wavelength, the oscillation wavelength of which is a 2.0 xcexcm band, whereas the optical fiber amplifier corresponds to an optical fiber amplifier with employment of an Tm3+ ion-doped optical fiber having a gain in the 2.0 xcexcm band.
In the coherent laser radar apparatus according to the first aspect, the CW laser corresponds to a CW laser radiation source having a high spectrum purity, in which a line width of oscillated laser radiation is made narrower than a width of a frequency dispersion which is applied to the received radiation and is caused by the target.
In the coherent laser radar apparatus according to the first aspect, the CW laser radiation source having the high spectrum purity includes: a solid-state laser module oscillated having a single wavelength, which contains a means for fine-controlling all, or any one of a temperature, a resonator length, and pumping intensity; and phase noise compensating means for detecting phase noise of output radiation from the solid-state laser module by using a portion of the output radiation thereof, and for supplying a feedback signal used to fine-control all, or any one of said temperature, said resonator length, and the pumping intensity from the phase noise detection output to the solid-state laser module.
The coherent laser radar apparatus according to the first aspect is featured by further comprising: means for detecting a time change in a summation between an optical path length of the transmitted radiation and an optical path length of the received radiation, and also for detecting a time change in a difference between an optical path length of the local radiation and the optical path lengths of the transmission/received radiation; and means for controlling the optical path length of any one of the transmitted radiation, the received radiation, and the local radiation based upon the output signal derived from the means for detecting the time change in the difference of the optical path lengths.
In the coherent laser radar apparatus according to the first aspect, the means for detecting the time change in the difference of the optical path lengths is arbandd by: a fixed delay device for applying a constant time delay to a portion of the pseudo-random modulation signal generated from the pseudo-random modulation signal generator; a correlating device for multiplying the output signal of the photodetector by the time-delayed modulation signal derived from the fixed delay device; a microwave reference oscillator having a high spectrum purity; and means for detecting a phase difference between the output signal of the correlating device and the output signal of the microwave reference oscillator.
In the coherent laser radar apparatus according to the first aspect, the means for controlling the optical path length is arbandd by: an electro-optical crystal element positioned in an optical path; and means for applying an electric field to the electro-optical crystal element.
In the coherent laser radar apparatus according to the first aspect, the respective structural elements of the coherent laser radar apparatus are set on a mobile object such as an aircraft, a satellite, and a vehicle.
In the coherent laser radar apparatus according to the first aspect, the pseudo-random signal generator generates a pseudo-random signal as a modulation signal in such a manner that a time width xe2x80x9cxcfx84xe2x80x9d of the pseudo-random signal per 1 bit is set to (xcfx84 greater than 1/fL) in the case that a minimum frequency of a measuring frequency band to be measured is selected to be xe2x80x9cfL.xe2x80x9d
In the coherent laser radar apparatus according to the first aspect, the coherent laser radar apparatus includes at least one set of such a combination between a fixed delay device and a correlating device, instead of both the variable delay device and the correlating device; the fixed delay device applies a constant time delay to a portion of the pseudo-random modulation signal of the pseudo-random modulation signal generator; and the correlating device multiplies the output signal derived from the photodetector by the output signal derived from the fixed delay device.
In the coherent laser radar apparatus according to the first aspect, the coherent laser radar apparatus is further comprised of an optical space communication apparatus including: an optical switch provided in a reception optical path from the optical antenna, for switching the optical paths; an optical receiver for receiving received radiation of an optical communication from the optical switch via an optical circulator; an optical transmitter for outputting transmitted radiation for the optical communication via the optical circulator to the optical switch; and a communication signal processing apparatus for demodulating the optical communication received radiation received from the optical receiver so as to extract communication information therefrom, and also for producing a modulation signal based upon communication information to be transmitted so as to output the produced modulation signal to the optical transmitter; and wherein: the optical antenna functions as an optical antenna for an optical communication purpose by switching the optical path by the optical switch, whereby a function of an optical space communication is additionally provided with the coherent laser radar apparatus.
In the coherent laser radar apparatus according to the first aspect, the coherent laser radar apparatus is further comprised of: a communication signal processing apparatus for demodulating the received optical communication received radiation so as to extract communication information therefrom, and also for producing a modulation signal based upon communication information to be transmitted so as to output the produced modulation signal; and wherein: the optical modulator modulates transmitted radiation for an optical communication by the modulation signal outputted from said communication signal processing apparatus; and a function of an optical space communication is added to the coherent laser radar apparatus by employing a function of an optical transmitter in which optical communication transmitted radiation is outputted by said CW laser and the optical modulator; a function as an optical communication optical antenna to the optical antenna; and a function as an optical receiver for receiving the optical communication received radiation to the photodetector.
Also, a coherent laser radar apparatus, according to a second aspect of the present invention, is featured by comprising: as an optical component, a CW laser oscillating a laser radiation having a single wavelength; a dividing means for dividing laser radiation derived from the CW laser; an optical modulator for modulating one of the laser radiation divided by the dividing means; an optical antenna for projecting the modulated laser radiation as transmitted radiation toward a target, and also for receiving scattered radiation from the target as received radiation; combining means for combining the other of the laser radiation divided by the dividing means as local radiation with the received radiation received from the optical antenna; and a photodetector for optical-heterodyne-detecting the combined radiation; and also comprising: as an electric component, a pseudo-random modulation signal generator for supplying a pseudo-random modulation signal to the optical modulator; a time delay device for time-delaying a portion of the pseudo-random modulation signal generated from the pseudo-random modulation signal generator; a correlating device for multiplying the output signal of the photodetector by the pseudo-random modulation signal time-delayed by the time delay device; and signal processing means for acquiring physical information such as a distance of the target and a velocity of the target based upon a intensity and a frequency of the output signal from the correlating device, and also the delay time set by the time delay device; wherein: the pseudo-random modulation signal generator employs as a pseudo-random modulation signal, such a pseudo-random sequence (PN code) that time required for one sequence length is sufficiently longer than time required for light which is reciprocated within a distance band where received radiation having sufficiently high intensity can be obtained by the optical antenna.
Furthermore, a coherent laser radar apparatus, according to a third aspect of the present invention, is featured by comprising: as an optical component, a CW laser oscillating a laser radiation having a single wavelength; a dividing means for dividing laser radiation derived from the CW laser; an optical modulator for modulating one of the laser radiation divided by the dividing means; an optical antenna for projecting the modulated laser radiation as transmitted radiation toward a target, and also for receiving scattered radiation from the target as received radiation; combining means for combining the other of the laser radiation divided by the dividing means as local radiation with the received radiation received from the optical antenna; and a photodetector for optical-heterodyne-detecting the combined radiation; and also comprising: as an electric component, a pseudo-random modulation signal generator for supplying a pseudo-random modulation signal to the optical modulator; a time delay device for time-delaying a portion of the pseudo-random modulation signal generated from the pseudo-random modulation signal generator; a correlating device for multiplying the output signal of the photodetector by the pseudo-random modulation signal time-delayed by the time delay device; and signal processing means for acquiring physical information such as a distance of the target and a velocity of the target based upon a intensity and a frequency of the output signal from said correlating device, and also the delay time set by the time delay device; wherein: the pseudo-random modulation signal generator owns a function capable of switching pseudo-random modulating signals produced based on a plurality of pseudo-random sequence (PN code) different from each other to thereby produce the switched pseudo-random modulating signal.